1. Field of the Invention
The present invention relates to gyroscopes, and more particularly, to a micro-electro-mechanical (MEM) gyroscope fabricated on a piezoelectric substrate.
2. Description of the Prior Art
An emergence of new consumer and automotive products that require angular velocity information has created an increasing demand for a smaller and inexpensive gyroscope or angular rate sensor. Rotating wheel gyroscopes, fiber optic gyroscopes and ring laser gyroscopes have been used extensively for inertial navigation and guidance systems; however, these gyroscopes are too bulky and expensive for newly emerging applications. A smaller and less expensive gyroscope is required for applications such as (i) automotive safety products, e.g., anti-skid system, ABS, airbag system, (ii) consumer products, e.g., 3-D pointer, camcorder, global positioning systems, sports equipment, (iii) industrial products, e.g., robots, machine control, guided vehicles, (iv) medical products, e.g., wheel-chairs, surgical tools, body movement monitoring, and (v) military products, e.g., smart ammunition, new weapon systems. A micro-electro-mechanical (MEM) sensor generally offers advantages of lightweight, small size, low power consumption, and low cost, particularly when manufactured using standard IC-fabrication techniques.
For any mechanical gyroscope, there is a stable reference vibrating motion (V) of a mass (m) such that an angular rotation (xcexa9) perpendicular to the direction of the vibrating motion (V) causes a Coriolis force perpendicular to the directions of both the vibrating motion (V) and the angular rotation (xcexa9), at the frequency of the vibrating motion (V). Therefore, the effect of the Coriolis force
F=2xc2x7mxc2x7Vxc3x97xcexa9
is a measure of the rate of the angular rotation (xcexa9).
A mass constrained by a stiffness element in a frame can be placed into an oscillatory motion in a z-direction by an input power source. If the frame is rotated about an x-axis, the oscillatory mass will experience a Coriolis force, in a y-direction, proportional to the applied rate of rotation. The Coriolis force acting on the mass attempts to cause a displacement of the mass in the y-direction proportional to the rate of rotation.
A conventional MEM gyroscope is a silicon-based vibratory sensor that utilizes an energy transfer between two vibrating modes of a mechanical structure. To achieve high sensitivity when subjected to a rotation, he energy from the vibrating modes must be efficiently transferred at a high Q level from an exciting direction to a sensing direction. A considerable effort is required to design and fabricate the vibrating structure and its support electronics to achieve a resolution of less than one degree per second.
Conventional MEM gyroscopes suffer from an inherent performance limitation because of their underlying operating principle, which is based on a vibration of a suspended mechanical structure, i.e., a comb structure, a beam, a disk, or a ring structure. It is often difficult and expensive to design and fabricate a mechanical structure with matching resonant frequencies of the two vibrating modes. The cost of the final product may also increase due to a need for electronic circuitry for controlling and detecting the status of the vibrating structure in order to improve dynamic range. In addition, the suspended vibrating mechanical structure is susceptible to external shock and vibration that occurs a frequencies not far removed from the frequency at which the gyroscope operates. Such disturbances can influence the vibrating structure. Consequently, the structure cannot be rigidly attached to the substrate for its resonant vibration, thereby also limiting its dynamic range.
For example, in a vibratory gyroscope such as a tuning fork gyroscope, the tuning fork consists of two tines connected to a junction bar. In operation, the tines vibrate at a designed frequency. When the tuning fork is subjected to rotation at its sensitive axis, a Coriolis force causes an orthogonal vibration. The Coriolis force can be detected from a differential bending of the tines. The tines and the vibrating structures are susceptible to external shock and vibration that occurs at frequencies close to the tine vibration frequency. Such disturbances can influence the vibrating structure and produce erroneous results. This is a major draw back of a vibrating gyroscope.
U.S. Pat. No. 6,003,370 to Yukawa et al. (hereinafter xe2x80x9cthe Yukawa et al. patentxe2x80x9d), entitled ELASTIC SURFACE WAVE GYROSCOPE, relates to an elastic surface wave gyroscope for detecting a Coriolis force generated on the surface of a piezoelectric substrate by the interaction of a surface oscillation caused by an elastic surface wave of the piezoelectric substrate and a rotary motion of the piezoelectric substrate. Two transducers on a piezoelectric substrate generate two elastic surface waves of different frequencies. Two pairs of reflectors for reflecting the two elastic surface waves produce two different standing waves. The two standing waves interfere with one another and produce an interference wave adapted for detecting the Coriolis force. Another pair of transducers fixed between the first two transducers will detect waves produced by the Coriolis force.
One disadvantage of the gyroscope of the Yukawa et al. patent is its susceptibility to cross coupling between the frequencies of the first two transducers. One technique for reducing cross coupling is to design the transducers to operate in a very narrow frequency band. Narrow band operation can be achieved by using a large number of IDTs, but unfortunately this increases the size of the gyroscope. Another alternative is to design the two transducers to operate with a very large difference in frequency. However, one transducer may be susceptible to harmonics of the frequency of the other transducer. For example if the devices are designed for 40 MHz and 80 MHz, then the 80 MHz device will receive the 40 MHz frequency because its harmonic frequencies are 80 MHz, 120 MHz, 160 MHz . . . and so on.
Another disadvantage of the gyroscope of the Yukawa et al. patent involves the arrangement of the third transducer between the first two. The third transducer can receive harmonic frequencies from the first two transducers and produce a high output signal even without rotating. Also, during operation of the gyroscope, the signal due to rotation will be small and difficult to detect in the presence of the first and the second classic surface waves.
It is an object of the present invention to provide a gyroscope with an improved capability for sensing a Coriolis force.
It is another object of the present invention to provide such a gyroscope that is inherently shock-resistant.
It is a further object of the present invention to provide such a gyroscope that is manufactured by a standard integrated circuit fabrication technique.
These and other objects of the present invention are achieved by a gyroscope that includes a piezoelectric substrate having a surface; a resonator transducer disposed on the surface, for creating a first surface acoustic wave on the surface; a pair of reflectors disposed on the surface, for reflecting the first surface acoustic wave to form a standing wave within a region of the surface between the pair of reflectors; a structure disposed on the surface within the region, wherein a Coriolis force acting upon the structure creates a second surface acoustic wave; and a sensor transducer disposed on the surface orthogonally to the pair of reflectors, for sensing the second surface acoustic wave.
A gyroscope in accordance with the present invention is a micro-electro-mechanical (MEM) gyroscope that utilizes a surface acoustic wave resonator (SAWR) and a surface acoustic wave sensor (SAWS). The SAWR and the SAWS are orthogonal to one another on a piezoelectric substrate, with a plurality of metallic dots arranged in an array therebetween. The metallic dots, which serve as a proof mass, are subjected to a reference vibration for detecting a Coriolis force. Experimental observations show that the sensitivity of the gyroscope is significantly improved by the presence of the metallic dot array. Since the SAWS are orthogonal to the SAWR, the SAWS directly receive only a relatively small portion of the signal generated by the SAWR, and accordingly, the surface wave produced by the Coriolis force can be easily detected. Also, the SAWR and SAWS operate at the same frequency, and thus one will not be adversely affect by the harmonic frequencies of the other.
A gyroscope in accordance with the present invention can be operated as a wireless gyroscope, or more precisely a passive gyroscope, for which a local power source is not necessary An antenna attached to the gyroscope receives an RF signal, which is coupled to a microstrip duplexer, and thereafter to the resonator IDT to initiate an oscillation thereof. The resonator IDT converts the RF signal to a first surface acoustic wave that is reflected between a pair of reflectors, thus forming a standing wave. Metallic dots at the anti-nodes of the standing wave are subject to an oscillation, which is used as a reference vibration for the gyroscope. When this gyroscope is subjected to rotation, a second surface acoustic wave will be generated with same frequency as the first surface acoustic wave and will propagate towards the sensor IDT. An output from the sensor IDT is coupled to the microstrip duplexer, and thereafter to the antenna for transmission to an external device that will process the signal to characterize the Coriolis force.
Also in accordance with the present invention, the gyroscope can offer greater sensitivity by providing a differential signal. That is, the gyroscope provides a first signal representing the effect of the Coriolis force on an array of metallic dots, and a second signal representing the effect of the Coriolis force on a region of the gyroscope that does not Include the array of metallic dots. The difference between the signals is processed to characterize the Coriolis force.